I have a classic NEXUS setup on my boat. The depth readout was working but last Wed night it stopped working. Just dashes. Checked it out and can't see anything that looks bad. It is the older type that has an amplifier for the depth. Checked the voltage at the CPU. Green 11.83, White 7.70, Yellow 4.88,checked the voltage at the amplifier output, the same. House battery 12.24. any ideas?